series vs wave train

series and wave train both are nouns.

Word	Noun	Adjective	Verb	Adverb
series	Yes	No	No	No
wave train	Yes	No	No	No

As nouns, wave train is a hyponym of series; that is, wave train is a word with a more specific, narrower meaning than series:

series (noun)	wave train (noun)
(mathematics) the sum of a finite or infinite sequence of expressions	a succession of waves spaced at regular intervals
a periodical that appears at scheduled times
a serialized set of programs
(sports) several contests played successively by the same teams
similar things placed in order or happening one after another
a group of postage stamps having a common theme or a group of coins or currency selected as a group for study or collection
(electronics) connection of components in such a manner that current flows first through one and then through the other